I went on a semi-deep dive for fun so bare with me lol.
It does seem like the name Nanaba has very little digital footprint attached. It could be one of many names the artist goes by. I was able to find this Bl that has the cover illustrator credited as Nanaba at the bottom left. Based on auto translate:
Author: Velvet Hammer (AreYOUok)
Cover Art......
While I do agree that heteronormative ideals in BL is a problem, I think another reason authors might do sudden m-preg is simply because they want or need to get more chapters out. I think this is especially true when the m-preg comes out of nowhere as if it wasn't planned at all. Either 1.) They're milking the story as much as they can and decided......